Viruses have been suggested or demonstrated to have utility in a variety of applications in biotechnology and medicine on many occasions. Each is due to the unique ability of viruses to enter cells at high efficiency. This is followed in such applications by either virus gene expression and replication and/or expression of an inserted heterologous gene. Thus viruses can either deliver and express genes in cells (either viral or other genes) which may be useful in for example gene therapy or the development of vaccines, or they may be useful in selectively killing cells by lytic replication or the action of a delivered gene in for example cancer.
Herpes simplex virus (HSV) has been suggested to be of use both as a gene delivery vector in the nervous system and elsewhere and for the oncolytic treatment of cancer. In both applications the virus must however be disabled such that it is no longer pathogenic but such that it can still enter cells and perform the desired function. Thus for non-toxic gene delivery to target cells using HSV it has become apparent that in most cases immediate early gene expression must be prevented/minimised from the virus. For the oncolytic treatment of cancer, which may also include the delivery of gene(s) enhancing the therapeutic effect, a number of mutations to HSV have been identified which still allow the virus to replicate in culture or in actively dividing cells in vivo (e.g. in tumours), but which prevent significant replication in normal tissue. Such mutations include disruption of the genes encoding ICP34.5, ICP6 and thymidine kinase. Of these, viruses with mutations to ICP34.5, or ICP34.5 together with mutations of e.g. ICP6 have so far shown the most favourable safety profile. Viruses deleted for only ICP34.5 have been shown to replicate in many tumour cell types in vitro and to selectively replicate in artificially induced brain tumours in mice while sparing surrounding tissue. Early stage clinical trials have also shown their safety in man.
However, while promise has been shown for various viruses including HSV for gene delivery/therapy or for the oncolytic treatment of cancer, the majority of this work has used virus strains which have been maintained in tissue culture cells for many years. In applications where the virus merely needs enter cells to deliver genes this may not prove problematical as maintenance in cell culture also requires the virus to enter cells, albeit often cells of a different type or species in comparison to the likely target cells for a vector. However, in applications where other properties are required, the use of laboratory virus strains may not allow the full potential of a virus in a particular application to be utilised.
HSV has the unique ability amongst viruses currently under development as vectors in that it has naturally evolved to infect and remain latent in neurons. HSV has also evolved to be highly efficiently transported along nerves from the site of infection, usually at the periphery, to the neuronal cell body, usually in the spinal ganglia. Such capabilities are not required in cell culture and as such capabilities require specific evolved properties of HSV, further adaptation to growth in culture may have resulted in optimally efficient axonal transport capabilities to have been lost. HSV vectors for gene delivery to the central or peripheral nervous system are likely to show maximum effectiveness if axonal transport properties have been retained at maximum efficiency. Here, inoculation at a peripheral site would then allow maximally efficient gene delivery to peripheral neuron cell bodies, and inoculation in the brain would allow maximally efficient gene delivery to multiple connected sites. Current vectors based on laboratory strains of HSV may not allow this to occur at the maximum efficiency possible. Indeed, because of HSV's high capacity to be transported along nerves, there is potentially a particularly large discrepancy between the properties which it is desired to conserve and those likely to be retained in culture.
HSV and other viruses such as adeno- or rheovirus also have potential utility in the oncolytic treatment of cancer. However, again viruses under development for such purposes have previously been extensively maintained in culture. As the oncolytic treatment of cancer requires active replication in often relatively slowly growing human tumour cells, it would be anticipated that adaptation of laboratory virus strains to growth in particular cultured cells may have reduced the efficiency with which such lytic replication in human tumour cells, or infection of human tumour cells, could optimally occur.